1. Field of the Invention
This invention relates to a system for remote monitoring a bidirectional digital or analog transmission line between two terminal stations and, more particularly, the detection of faults along the line in order to regulate the gain of the intermediate amplification circuits, such as repeaters, along the line. It generally relates to selective remote location of the intermediate amplification circuits, the looping of the pairs of amplification circuits concerned in both directions of transmission, and regulation of the gains of the intermediate amplification circuits.
2. Description of Prior Art
Generally, the intermediate amplification circuits in a transmission line are each provided with regulating circuits for regulating the amplification gain and are distributed in pairs along the line between the terminal stations which are termed herein as a monitoring station and a monitored station. The monitoring station comprises means whereby remote location signals of the pairs of amplification circuits are transmitted from the monitoring station to the monitored station over the forward transmission channel of the line, means for transmitting at least one test signal over the forward transmission channel, and means for detecting the faults in the information signal normally transmitted over the backward transmission channel from the monitored station to the monitoring station and/or in the test signal after a pair of amplification circuits has been looped through the monitoring station. Each pair of forward and backward intermediate amplification circuits comprises switching means whereby the output of the forward amplification circuit is connected to the input of the backward amplification circuit by looping through the monitoring station, and means for detecting said remote location signals in order to control the switching means.
Numerous remote monitoring system are known for a transmission line and are distinguished from one another by their methods for the remote location of the line repeaters and, specifically, by the type of remote location signals.
German Patent Application P No. 1915867.9 relates to the remote location of pairs of repeaters in a transmission line of the PCM type. The individual address of each repeater pair is obtained by means of a predetermined voltage level of a control signal which is transmitted by the monitoring station on the forward transmitted channel of the line. A clipper is included in the locating circuit belonging to the repeater pair whose location is to be determined, and detects only the predetermined voltage level in order to carry out the single looping of the output of the forward repeater to the input of the backward repeater.
Also, and contrary to the recommendations of the Consultative Committee International Telegraph and Telephone (C.C.I.T.T.), the test pulse signal transmitted by the monitoring station according to the German Patent Application P No. 1915867.9 is at the same frequency of those normally transmitted in the transmission line. This means that the remote location of the repeaters can be carried out only after stoppage of the normal data transmission on the line.
Swiss Patent Application No. 608151 also relates to a selective remote location of a pair of repeaters in a PCM digital line. Each pair of repeaters is remote located by means of a specific address word which is transmitted from the monitoring station over the forward transmission channel of the line and which is detected in a monitoring circuit of the repeater pair. The remote location procedure also comprises single looping of the output of the forward repeater to the input of the backward repeater.
Another remote monitoring system of the kind defined hereinabove is described in U.S. Pat. No. 4,187,415, issued Feb. 5, 1980. This system relates to data transmission lines, such as telephone channels, in analog form or digital form of the PCM type. For remote locating a pair of intermediate amplification circuits, after detection of a transmission fault in the line, either by the monitoring station or by the monitored station, a first remote location signal comprising low-frequency modulated pulses, e.g. in the case of a cable link, is continuously transmitted by the monitoring station over the forward transmission channel in order to effect the above-mentioned connection of all the pairs of intermediate amplification circuits. At the pre-looping stage, only the pair of intermediate amplification circuits immediately adjacent the monitoring station is located. The input and the output of two successive intermediate amplification circuits on the backward transmission channel are disconnected in such remote location so that the interruption of the data signal transmission over the two channels must then be signalled downstream of the monitored station on the forward channel and downstream of the monitoring station on the backward channel. If the transmission fault is not detected again after a test signal of a type similar to that of the data signal, a succession of second remote location signals modulated at a frequency different from that of the first remote location signal is transmitted on the backward channel in the forward direction. Each second remote location signal enables the previously looped pair of amplification circuits to be unlooped and the immediately following pair to be looped through the monitoring station. For remote locating any pair of intermediate amplification circuits these successive simultaneous looping and unlooping processes have to be repeated from the amplification circuit pair adjacent the monitoring station. Stoppage of the transmission of the first remote location signal causes the unlooping of all the pairs of intermediate amplification circuits.
Another remote location system for a digital data transmission line uses solely low-frequency remote location signals, i.e. signals which can be mixed with the data signals, and is disclosed in an article by Frederic PLATET et al, published in the French periodical "Cables & Transmission", volume 29, January 1975, No. 1, pages 69 to 108. In this system, a single remote location signal at the low-frequency of 500 Hz is transmitted on the forward transmission channel of the line. During the continuous transmission of this signal, the first pair of intermediate amplification circuits, i.e. repeaters, passes into the loop position and interrupts the transmission of the remote location signal. A short interruption of the remote-location signal results in unlooping of the first pair of repeaters and restores the transmission to the next forward repeater, which is looped to the associated backward repeater when transmission of the remote location signal is restored. When the interruption of the remote location signal lasts for a sufficiently long period, all the pairs of repeaters are unlooped.
In the latter two systems of remote monitoring, it appears that in order to carry out remote location of any pair of intermediate amplification circuits, it is necessary to proceed step-wise, or more specifically to loop and unloop all the repeater pairs between the monitoring station and the repeater pair in question. Also, remote location of a repeater pair for the purposes of analyzing its insertion losses or transmission losses, adversely requires interruption of the normal transmission of the data signals on the line.
Also, in the four remote monitoring systems referred to hereinabove, the terminal monitoring station is adapted only to detect the faults and in no case it is able to modify the gain of each intermediate amplification circuit independently of the others.
The reason for this is that an intermediate amplification circuit of this kind is designed with a negative feedback loop whereby the gain of the intermediate amplification circuit can be modified automatically from a pilot signal which is transmitted continuously in the line. It is known that such local automatic gain regulation is required in order to correct the attenuation of the data which is caused by each section of the line. Apart from insertion losses of the line, the causes of this attenuation lie with temperature variations, variations in the supply voltage, ageing of the repeater components, and so on. Each gain regulating circuit associated with an intermediate amplification circuit is designed so that the noise in the line is at a minimum when the line is "aligned", i.e. when the contribution of each regulating circuit results in the same level at its channel output as the level initially transmitted to the input of the channel on the sending station side. With regard to the general construction of a gain regulating circuit of this kind reference may be made to an article by Henri SOULIER, published in the French periodical "Annales des Telecommunications", volume 29, No. 11-12, November-December 1974, and to paragraph 2.4.2. of the article by C. CHALHOUB and P. FRANCO, published in the French journal "L'onde electrique", volume 51, pages 118 to 127, February 1971.
A gain regulating circuit generally comprises a band-pass filter which samples a pilot signal at the output of the amplification circuit. The pilot signal is amplified and rectified, then compared with a reference voltage. The level difference arising out of this comparison enables the gain of the amplification circuit to be corrected by acting on a variable equalization network which comprises one or more variable elements. These variable elements are inserted in the negative feedback loop of the amplification circuit and are varicap diodes or thermistors, for example. In a regulating circuit of this kind, the amplification gain variation curve must be a function of the attenuation due to the nature and the length of the channel section upstream of the amplification circuit. Consequently, before it is used, the topography of the transmission line between the terminal stations and the nature of the line sections must be accurately known if the level of the reference voltage is to be suitably selected for each regulating circuit.
To obviate this disadvantage, it has been proposed to interconnect between the input of each intermediate amplification circuit and the corresponding upstream channel section a variable section simulator such that the system comprising the section plus the simulator has a fixed attenuation curve which is equalized in the amplification circuit. A simulator of this kind is described, for example, in the article by Y. SAMOEL, pages 174 to 183 and in the article by F. PLATET et al, pages 332 to 371, published in the French periodical "Cables & Transmission", special issue, December 1975. This organization enables all the output signals of the intermediate amplification circuits to be positioned at the same level by comparison with a same predetermined reference voltage.
Other gain regulating circuits are based on measuring the temperature at the location of the intermediate amplification circuit on the hypothesis that the temperature variations at that point of the line are identical to those of the upstream line section, but this is not always true. Since the line is perfectly aligned when it is put into service, the temperature variation data is converted to a variation of the attenuation of the line section.
In remote monitoring systems using such gain regulating circuits, the monitoring terminal station does not have separate data concerning the gain of each intermediate amplification circuit. For example, after a change of line section associated with an amplification circuit or after associated modifications such as the insertion of other additional amplification circuits in a repeater relay or unit for the commissioning of a new line included in one and the same cable as the line in question, the reference voltage or the variable elements of each regulating circuit must be corrected in situ in order to re-align the line.